Horizon sensors are mounted in satellites, spacecraft and the like to provide attitude information (orientation) of the orbiting body with respect to the earth. A detector in the horizon sensor, usually an infrared detector, is scanned or oriented across the earth's horizon which is extremely warm when compared to cold outer space generating a signal representing a line of thermal discontinuity.
A major problem with horizon sensing arises from the large variations in earth radiance with latitude, season and weather. Even though the horizon profiles are most uniform in the 14-16 micron spectral region (carbon dioxide absorption band), the profiles still vary with latitude, season and weather. Significant longitudinal radiance variations also occur in the winter hemisphere due to storms, extra tropical cyclones, frontal systems, etc.
Several compensation techniques have been employed in an effort to remedy this problem, eg. a compensation detector viewing the earth in the vicinity of the sensing detector is provided in accordance with U.S. Pat. No. 3,486,024. However, this assumes that the radiation is uniform over both the "sensing" and "compensation" detectors which is not always the case. Since the horizon sensor is positioned above the earth and at altitudes ranging from 200 nautical miles to above geosynchronous 22,000 nautical miles, the latitudinal and longitudinal radiance variations over the sensing and compensating fields can produce substantial pitch and roll errors. These errors also occur at lower altitudes.
Additional pitch and roll errors can also result from thermal gradients in the horizon sensor per se, for example, temperature gradients in the detector mount, filter, filter mount, lens, etc.